1. Field of the Invention
The present invention relates to a wire electrical discharge machine and, more particularly, to a wire electrical discharge machine capable of correcting positional displacement (thermal displacement) of a wire electrode that is caused by thermal displacement of the machine.
2. Description of the Related Art
FIG. 5 is a schematic diagram illustrating a prior art wire electrical discharge machine. The wire electrical discharge machine machines a workpiece 29 by causing an electrical discharge between a wire electrode 1 and the workpiece 29.
The workpiece 29 placed on a workpiece mount is machined while it is immersed in a machining fluid in a work tank 6. The machining fluid including machined cuttings generated during machining is discharged from the work tank 6 to a wastewater tank 26 through a pipeline (not shown). The machining fluid collected in the wastewater tank 26 is filtered to remove the machined cuttings and then transferred to a freshwater tank 27. The freshwater tank 27 is provided with a machining fluid temperature regulator 28. The machining fluid temperature regulator 28 adjusts the temperature of the machining fluid stored in the freshwater tank 27 by pumping up the machining fluid from the freshwater tank 27 through a piping 37 and passing it through the machining fluid temperature regulator 28. The temperature-adjusted machining fluid is pumped up from the freshwater tank 27 by a pump (not shown) and fed back to the work tank 6 through pipelines 35, 36.
The positional relationship between the wire electrode 1 and the workpiece 29 is adjusted by a control unit 24 that drives motors for individual shafts. An X-axis drive motor 17 adjusts the relative positional relationship by moving an X-axis saddle 11, and a Y-axis drive motor 18 adjusts the relative positional relationship by moving a Y-axis saddle 10, to adjust the relative positional relationship between the wire electrode 1 and the workpiece 29. A U-axis drive motor 20 and a V-axis drive motor 21 determine the tilt of the wire electrode 1. The U-axis drive motor 20 determines the tilt of the wire electrode 1 by moving a U-axis saddle 13 to reposition an upper head section 7. The V-axis drive motor 21 determines the tilt of the wire electrode 1 by moving a V-axis saddle 14 to reposition the upper head section 7. A Z-axis drive motor 19 determines the vertical position of the upper head section 7.
As described above, the positional relationship between the wire electrode 1 and the workpiece 29 is adjusted by combining a plurality of mechanical sections. Position coordinates of each axis that are required for positional control of each axis are detected by a position detector built-in a motor for each axis.
The wire electrical discharge machine is thermally deformed when a change occurs in a temperature environment in which it is installed, in the temperature of a mechanical section, or in the temperature of the machining fluid. Such thermal deformation causes thermal displacement in which the position and tilt of the wire electrode are displaced from the position and tilt commanded for machining purposes. If the position and tilt of the wire electrode are displaced, the positional relationship between the wire electrode 1 and the workpiece 29 changes to decrease the accuracy of machining. As such being the case, when high machining accuracy is required, it is necessary to perform machining not to cause the thermal displacement while the wire electrical discharge machine is installed in a temperature-controlled environment such as a thermostatic chamber.
However, controlling the temperature of a machine installation environment requires the use of a temperature control facility, that is, involves a large amount of investment and operating cost. In reality, therefore, many users cannot afford to use a temperature control facility.
Japanese Patent Application Laid-Open No. 61-297057 or No. 7-75937 discloses a thermal displacement correction function that provides against thermal displacement of a machine tool by acquiring temperature information with temperature detectors disposed in various sections of a machine, computing a correction value for thermal displacement, and subjecting each axis to drive control based on the computed correction value in order to suppress relative positional displacement between the wire electrode 1 and the workpiece 29. When the above-described thermal displacement correction function is used to give an appropriate correction value, the relative positional relationship between the wire electrode 1 and the workpiece 29 remains unchanged even in an environment where the temperature changes. This makes it possible to suppress a decrease in the machining accuracy.
However, mechanical components of the wire electrical discharge machine are assembled from a plurality of mechanical elements and formed of different types of members. Further, as the environment in which the wire electrical discharge machine is installed varies from one user to another, it is difficult to set a thermal displacement correction value suitable for all temperature environments. A discrepancy occurs between a correction value preset for thermal displacement and an actual thermal displacement of the machine depending on the temperature environment and on the difference between one wire electrical discharge machine and another. In some cases, therefore, appropriate corrections cannot be made.
It has been difficult for the users to adjust the difference between a preset correction value and actual thermal displacement and compute a correction value appropriate for a user-specific temperature environment because it has been necessary, for example, to make measurements with an accurate measuring instrument, install a measurement sensor for making measurements, and rewrite a complex correction value computation program.